WO2007006983A1

WO2007006983A1 - Polarizer and corresponding projector

Info

Publication number: WO2007006983A1
Application number: PCT/FR2006/050285
Authority: WO
Inventors: Pascal Benoit; Jean-Jacques Sacre; Khaled Sarayeddine; Gunther Haas
Original assignee: Thomson Licensing
Priority date: 2005-07-07
Filing date: 2006-03-31
Publication date: 2007-01-18

Abstract

The invention concerns a polarizer (32) comprising means for radial polarization (101 to 103) of an incident beam and a projecting device using said polarizer. Thus, the invention enables in particular parasite reflections on a back-projection display to be limited.

Description

Polarizer and corresponding projector.

1. Field of the invention

The invention relates to the field of image projection. More specifically, the invention relates to video projectors of the type front projector or backlight, shallow.

2. Technological background.

According to the state of the art, projectors have the disadvantage of being relatively bulky.

In retro-projection systems (especially thin), the imaging system used to enlarge the image is said to be very wide-angle. The output angle of the imaging system is of the order of 70 degrees. A transmissive screen which materializes the image is translucent and also has the function of collimating the beam emanating from the imaging system before broadcasting it. This screen then receives a non-polarized incident beam or polarization in a given direction parallel to an edge of the screen, with an angle of incidence of the order of 70 °.

Such systems have the disadvantage of having significant losses of flux and uniformity after refraction of the beam in the Fresnel screen. These losses are detrimental to the quality of the image and thus limit the application of the thin projector concept.

3. Summary of the invention.

The invention aims to overcome these disadvantages of the prior art.

More particularly, the invention aims to improve the performance of the imaging beam and / or the uniformity of the projected image. For this purpose, the invention proposes a polarizer, which comprises means for radial polarization of an incident beam. A polarizer is understood in the broad sense; it is an optical element capable of selecting a specific polarization or of an optical element capable of rotating a polarization or of an optical element capable of both selecting and rotating a polarization.

According to a particular characteristic, the radial polarization means comprise means for rotating the polarization of a polarized beam, so that the light is polarized radially at the output of the rotation means (when the light is polarized at the input). Thus, polarization rotation means produces a radial polarization of an incident beam polarized in a determined direction.

Advantageously, the radial polarization means comprise a birefringent layer. According to a particular characteristic, the birefringent layer is in liquid crystal.

According to an advantageous characteristic, the birefringent layer has a helical structure.

Advantageously, the birefringent layer comprises: an input surface with an optical axis parallel or perpendicular to a first direction; and an output surface with an optical axis parallel or perpendicular to the direction of the polarization of the output light. According to another characteristic, the birefringent layer has a homogeneous structure according to its thickness.

According to one particular characteristic, the birefringent layer comprises an input surface and an output surface with an optical axis of the ellipsoids of the indices of the median liquid crystal molecules between an input bias in a predetermined direction and a substantially radial output polarization. .

Advantageously, the radial polarization means comprise linear polarization means positioned before the means for rotating the polarization of a polarized beam. According to one particular characteristic, the radial polarization means comprise means for selecting a radial polarization.

Advantageously, the radial polarization means comprise means for selecting or rotating the polarization so that, at each point at the output of the means, the light is polarized at its output along a polarization direction which makes an angle less than or equal to 15 ° with the plane that includes an optical axis of the incident beam and the incident ray at the point considered.

The invention also relates to a projector comprising an imaging source producing an imaging beam, and said polarizer so as to radially polarize the imaging beam. According to one particular characteristic, it comprises a Fresnel screen, the imaging beam illuminating said screen being polarized P at any point of incidence on said Fresnel screen.

4. List of figures.

The invention will be better understood, and other features and advantages will appear on reading the description which follows, the description referring to the appended drawings among which:

- Figure 1 schematically illustrates the projection principle according to the invention in a particular embodiment;

FIG. 2 shows the efficiency of a Fresnel lens with a polarized imaging beam according to the invention;

- Figures 3 and 7 show an overhead projector according to the invention in a particular embodiment; FIG. 4 illustrates the polarization according to a front view in the headlamps of FIGS. 1 and 3, and

- Figures 5 and 6 illustrate a polarizer implemented in the projectors of Figures 1 and 3.

5. Detailed description of the invention.

The general principle of the invention is therefore based on the use of an imaging beam with radial polarization so as to illuminate a Fresnel lens with a polarization P.

Figure 1 illustrates the principle of projection according to the invention in a projector 1 and shows how a light imaging beam 11 is refracted by the thin projection screen.

The projector 1 comprises a Fresnel screen 12 illuminated by the imaging beam 11 (including a beam 110). The incident beam 11 is first refracted by an input face 120 of the Fresnel screen and then by an output face 121 so as to be transmitted in a direction substantially normal to the screen 12 before being broadcast by a diffuser (not shown) to enlarge the viewing area horizontally and vertically.

FIG. 2 shows efficiency curves (corresponding to distinct polarizations) of an imaging beam (along the axis 28) as a function of the angle of incidence of the beam (along the axis 27): - polarization P (according to the invention) on the curve 20 (that is to say in the plane of incidence (plane formed by the beam radius passing through the point considered and the optical axis 13 of the system at the of the screen)) ; - S polarization on the curve 22 (that is to say normal to the plane of incidence); and

- average according to the curve 21 (corresponding to the state of the art). This figure shows how the transmission of the screen decreases with the angle of incidence for the two polarizations respectively parallel and perpendicular to the plane of incidence.

It can be seen in particular that for an angle of incidence close to 70 degrees the losses are of the order of 50% for a non-polarized beam (average between S and P). If the imaging beam is unpolarized, or if its polarization is perpendicular to the plane of incidence (on the screen itself), then a large part of the beam is reflected.

According to the invention, the efficiency with an angle of incidence equal to 70 ° remains close to 80% (represented by line 24). The possible amplitude of the angles of incidence (between a line 23 corresponding to a minimum incidence of approximately 10 ° which is conventionally found in fine overhead projectors and the line 24) illustrated by the arrow 25 is therefore very extensive according to the invention and thus allows to increase the angles of incidence and thus reduce the size of the retro-projectors or bring them closer to a projection screen (for frontal projection). Of course, there is no lower limit for the angle of incidence that can, according to the invention, be zero.

The invention thus solves the problem of losses by controlling the polarization state of the imaging beam before refraction on the screen. This invention therefore applies to projection systems already using a polarized beam by rotating the polarization and also relates to micro-display projection systems (or "micro-display" in English or imager) using polarized unpolarized light the light so as to select the polarization P.

According to the state of the art, the projection systems are either unpolarized or use a reflective retro-projection screen, where the total internal reflection is insensitive to polarization.

According to the state of the art, in conventional retro-projection systems, the angle of incidence on the screen is of the order of 45 degrees maximum as illustrated by the arrow 26. Here too, it is arranged that the polarization of the beam is parallel to the plane of the screen, polarizing the beam.

According to the invention, the polarization state of the imaging beam is modified before its refraction so as to obtain a polarized beam parallel to the incidence plane at each point of the screen. In this way, the losses are reduced to a minimum (about 15% instead of 50% for an incidence of 70 °). According to a particular embodiment of the invention, a polarization rotation plate is preferably placed after the projection lens. After this projection objective (in a thin projection configuration), the image is almost materialized, the optical fields being well separated. The blade has an important characteristic of rotating the polarization of the beam according to the position of the beam on the blade. This blade is a rotator of variable polarization. The variation is spatial in the plane of the blade. In the system of interest to us, the incident beam has substantially the same state of polarization input. At output the blade turns the polarization so that the incident beam on the projection screen is parallel to the incidence plane at each point of the screen. Figure 3 depicts a projector 3 which includes an imaging source (not shown) based on an imager (LCOS, transmissive LCD or DLP), a projection lens 30 producing an imaging beam 31 which passes through a rotator of polarization 32 to form an imaging beam 35, a folding mirror 33 and a screen 34. The polarization rotator 32 produces from a polarized imaging incident beam the radially polarized imaging beam 11 which is reflected. by the mirror 33 to backlight the transmissive screen 34.

Preferably, the rotator 32 is located immediately after the projection lens 30. According to an alternative embodiment of the invention, the polarization rotator is placed in the lens in an area where the optical fields are separated.

According to another variant, the polarization rotator is glued to the lens, before or after the lens. According to another variant of the invention using a transmissive LCD (or liquid crystal display) or DMD (or "micromirror component") micro-display. "Digital Micromirrors Device"), the polarization rotator is stuck to the micro-display. If a DMD is used, the incident beam is reflected and thus crosses twice the polarization rotator; in this case, a homogeneous birefringence structure will preferably be used, the polarization rotation carried out by crossing the rotator must be divided by two (for example by using a radially oriented λ / 4 blade). If a three-imager system (for example, triLCD system) is used, a polarization rotator is placed on each optical path. FIG. 7 schematically illustrates the overhead projector 1 without a folding mirror 33 (the folding of the beam 11 not being illustrated for the sake of clarity) and highlighting the angle of inclination β of the polarization rotator 32 with respect to a mean (central) radius 70 of the incident beam 11. Preferably, the angle of inclination the inclination angle β is between + or -20 °. According to other embodiments, the angle of inclination β is arbitrary (the rotator 32 being for example perpendicular to the optical axis 13 or to an extreme radius of the beam 11).

Figure 4 shows the polarization direction of the beam as it is with a correction blade (rotator 32). Thus, each of the radii 45 to 49 belonging to the beam 11 coming from a point situated on the optical axis 13 of the system has a respective polarization 40 to 44 radial when it strikes the screen 12.

FIG. 5 illustrates an exemplary embodiment of a blade (rotator 32) with a helical liquid crystal. The blade is formed by two transparent glass substrates (or two plastic blades) 101a and 102a, and a birefringent layer 103 (example a layer of liquid crystal or polymerized liquid crystal). The liquid crystal is anchored to the blade by means of pre-alignment (or friction) layers 101b and 102b respectively belonging to plates 101 and 102. These layers are aligned homogeneously on the first plate 101. They are aligned with each other. a non-homogeneous way on the second plate 102 in the plane of this plate. The birefringent layer 103 is in a helix (or TN of the English "twisted nematic").

Figure 6 shows the direction of friction (or pre-alignment) of the layers of the plates 101 and 102, distributed on each plate for better polarization rotation. According to the illustrated embodiment, to turn the polarization of an angle α (for an incident beam with a polarization parallel to the layer alignment of the layer 102b which corresponds to the polarization of the beam 31), it is necessary that the angle Φ between the polarization of the beam 31 and the alignment layers of the plate 102b is equal to α . The angle Φ between the friction (or pre-alignment) on the plate 101 and the direction of the polarization of the incident ray and the layers of the glass plates preferably varies from 0 ° to about 80 ° and preferably 90 ° on the plate 102 so that the polarization of the incident beam 31 is rotated from 0 to 80 ° and preferably 90 °. Preferably, the layers of pre-alignment of the plate

102, are carried out using a photographic process (for example by photopolymerization with polarized light) (for anisotropic pre-alignment) which is particularly simple to implement in order to obtain directions of polarization that vary according to the point of the plate considered. An example of such a process is described in the patent document EP1157302 filed in the name of RoNc AG and inserted here by reference (according to this method, monomeric liquid molecules or pre-polymer crosslinkable liquid are placed on a layer of material or mixed with the material to adopt a predetermined alignment, the material being crosslinked during unpolarized or circularly polarized radiation from an oblique direction which aligns the liquid crystal molecules). In the process described in patent document EP1157302, the direction of the anisotropy is determined by the plane of incidence of the ultraviolet to polymerize. According to the invention, it is possible to use other materials which are oriented by a linear polarization, which makes it easier to manufacture. According to an alternative embodiment, the pre-alignment layers of the plate 102 are made by friction or brushing of the anchoring layer, the direction of friction corresponding to the direction of polarization. According to yet another variant, the orientation of the anchoring layer is obtained by applying a lateral electric field on the plate 102 (with for example an electrode array placed on either side of the plate 102) . In general, the orientation of the anchoring layer can be obtained by any means creating anisotropy on the anchoring layer. For the plate 101, it is possible to use also photographically and preferably a priorization by friction (less expensive), the direction of friction being identical over the entire plate 101 (Φ = 0). When the layers 101 and 102 are made, the cell comprising the two substrates 101 and 102 is filled with the liquid crystal and sealed. The liquid crystal is polymerized or not. According to this embodiment, the liquid crystal 103 has a helical structure so as to have: an input surface 101 with an optical axis (optical axis of the ellipsoids of the indices of the liquid crystal molecules) parallel (respectively perpendicular) to the direction of polarization of the input light; and

an output surface 102 whose optical axis is parallel (respectively perpendicular) to the direction of the polarization of the output light.

In this case, the product of the index variation and the local thickness d of the plate divided by the wavelength of the incident beam is much greater than the quotient Φ divided by π (Δn.d / λ »Φ / π); thus, an achromatic effect of the blade is obtained. The index variation Δn corresponds to the index difference between the axes of each ellipsoid (birefringence of the medium in the liquid crystal). Preferably, the variation of index Δn and the thickness d are chosen so that the transmission (of the liquid crystal 103) which is a function of An / d corresponds substantially to the second or third maximum of Gooch-Tarry.

According to an alternative embodiment of the invention, particularly simple to implement and particularly well adapted to a reflective mode, (ie to the mode where the blade is mounted in front of a reflective modulator (in particular LCoS (liquid crystal on Silicon of the English "Liquid Crystal On Silicon") or DLP ® or DMD ® ("Digital Micromirror Device" of the Texas Instruments ® company)) (with, for example, a plastic substrate), the liquid crystal 103 has a homogeneous structure according to its thickness (unlike the helical structure) and the product of the index variation and the local thickness d of the blade divided by the length of the wave of the incident beam is 0.5 (Δn.d / λ = 1/2) (in this case, we have a chromatic effect) .In this variant, the optical axis of the ellipsoids of the indices of the liquid crystal molecules (or direction of friction) is identical on the two subst rats 102 and 101 and corresponds to α / 2 (the birefringent layer comprises an entrance surface and an exit surface with an optical axis of the ellipsoids of the indices of the liquid crystal molecules median between the input polarization and the radial polarization in exit). This optical axis is obtained by any means creating anisotropy on the anchoring layer (in particular by photopolymerization, friction or application of an electric field as indicated above). According to a simplified implementation, there is no anchoring layer, the polymerizable liquid crystal being oriented solely by the photopolymerization step. According to a preferred embodiment corresponding to this variant, the birefringence seen by an incident ray (or ellipsoid indices, indices seen by an incident ray depending on the angle of incidence on the polarization rotator at a given point) is taken into account. Thus, the inclination of the incident ray with respect to a normal polarization rotator is taken into account for the calculation of the local thickness of the plates 101 and 102. According to a simplified embodiment, an approximation of the indices for the polarization rotator is carried out. calculation of the thickness, without taking into account the inclination of an incident ray with respect to a normal polarization rotator. Preferably, according to the invention, the imaging beam is polarized radially when it meets the screen. Nevertheless, according to alternative embodiments, the invention does not require an exactly radially polarized light since a substantially radial light is sufficient to obtain a substantially uniform image. According to a variant embodiment with a liquid crystal with a homogeneous structure according to its thickness, a bilayer system is used, which makes it possible to have with a different dispersion relation for each of the layers, to have an independent polarization of the beam wavelengths. incident. According to various embodiments, the rotator allows rotation of the substantially radial polarization. Thus, the polarization obtained at the output of the rotator is such as the angle between the polarization direction and a plane which comprises the optical axis 13 of the beam and the incident ray at the point considered is preferably less than or equal to 15 ° and more preferentially at 5 °.

According to alternative embodiments of the rotator with helical structure of the liquid crystal or homogeneous structure, the variation of the direction of polarization at the output is not continuous by step. According to these variants, the apex angle of the considered zones (the vertex passing through the optical axis 13 of the incident beam) is preferably less than or equal to 15 ° and even more preferentially to 5 °. With a rotator with structure liquid crystal helix, the output plate has an anchor layer configured to obtain a polarization varying in steps.

In the previously described embodiments, it is assumed that the incident beam is polarized. Such embodiments are particularly implemented when the micro-display requires a polarized illumination beam. According to alternative embodiments, the incident beam is not polarized and a vertical or horizontal polarizer (and / or a polarization recovery device rotating the polarization of

90 ° undesired) is placed before the polarization rotator (for example just before or, on the contrary, well before the polarization rotator (the rotator being, for example, placed after the lens and the polarizer before the lens near the lens). micro-display).

According to other variants of the invention, according to which the imager does not require polarized light, the projector uses a direct selection component of a radial polarization (for example just before the micro-display). Such a component is, for example, a grid polarizer where each line of the grid is a circular arc with a center on the optical axis 13 of the beam. According to an alternative embodiment, the component is a piece-absorbing polarizer, angular sectors selecting a polarization in a given direction are defined: these sectors preferably have a point placed on the optical axis 13 of the beam and make it possible to obtain a substantially radial polarization.

Of course, the invention is not limited to the embodiments described above. In particular, the invention applies to any type of front projector or backlight with radially polarized beam.

According to the invention, the radial polarization means are not limited to those described but relate, on the contrary, any type of means for modifying the polarization to obtain a radially polarized imaging beam.

In addition, such means can be positioned in the projector before or after the imager.

The invention can also be applied for non-thin backprojection systems, this type of component can improve the transmission of the screen.

In this case, generally the optical axis of the imaging beam is centered on the axis of the screen. The polarizer (rotator or means of selection polarization) is then preferably of symmetry of revolution with respect to the center of the optical beam.

Finally for the projection systems used in the projection

3D (with two polarized projectors), again the imperfections of these systems can be improved without the addition of a polarizer (flux losses of at least 20%) by the addition of compensating blade for the direction of rotation of polarization.

According to some embodiments of the invention, the imaging beam is polarized only radially on the edges (where the polarized beams S are the least well transmitted, the angles of incidence being stronger) and not in the center. Thus, either only a portion of the imaging beam passes through polarization selection or rotation means to obtain a radial polarization, or, preferably, the polarization selection or rotation means comprise a neutral zone (ie, not to say radial).

According to another variant, the rotation or polarization selection means comprise at least two zones, a first zone making it possible to obtain a perfectly or substantially radial polarization on the edges, with a direction corresponding to the optical axis and a second zone. with a radial polarization which is not centered on the optical axis of the incident beam. This embodiment makes it possible to limit the variation on the spots in the center. The realization can be facilitated by ensuring continuity of the polarization between the two zones.

According to alternative embodiments, the blade for selecting a radial polarization or the polarization rotator are not planar. In particular, according to some embodiments of the invention, the blade or the rotator is a curved blade so that, at any point of the blade, each average radius of the incident beam is normal to the surface of the blade.

Claims

1. Polarizer (32), characterized in that it comprises means for radial polarization (101, 102, 103) of an incident beam.

2. Polarizer according to claim 1, characterized in that said radial polarization means comprise means for rotating the polarization of a polarized beam, so that the light is radially polarized at the output of said means.

3. Polarizer according to claim 2, characterized in that said radial polarization means comprise a birefringent layer (103).

4. Polarizer according to claim 3, characterized in that said birefringent layer is in liquid crystal.

5. Polarizer according to any one of claims 3 and 4, characterized in that the birefringent layer has a helical structure.

6. Polarizer according to claim 5, characterized in that said birefringent layer comprises: an input surface (101a) with an optical axis parallel or perpendicular to a first direction; and an output surface (102a) with an optical axis parallel or perpendicular to the direction of the polarization of the output light.

7. Polarizer according to any one of claims 3 and 4, characterized in that the birefringent layer has a homogeneous structure according to its thickness.

A polarizer according to claim 7, characterized in that said birefringent layer comprises an input surface and an output surface with an optical axis of the ellipsoids of the indices of the liquid crystal molecules median between an input bias in a predetermined direction and a substantially radial bias at the output.

9. Polarizer according to any one of claims 2 to 8, characterized in that said radial polarization means comprise linear polarization means positioned before said means for rotating the polarization of a polarized beam.

Polarizer according to claim 1, characterized in that said radial polarization means comprise means for selecting a radial polarization.

11. Polarizer according to any one of claims 1 to 10, characterized in that the radial polarization means comprise means for selecting or rotating the polarization so that, at each point at the output of said means, the light is polarized at an output in a polarization direction at an angle less than or equal to 15 ° with the plane comprising an optical axis of the incident beam and the incident ray at the point in question.

12. Projector (3) comprising an imaging source (30) producing an imaging beam (35), characterized in that it comprises a polarizer (32) according to any one of claims 1 to 11 so as to radially polarizing said imaging beam.

13. Projector according to claim 12, characterized in that it comprises a Fresnel screen (34), the imaging beam illuminating said screen being substantially polarized P at any point of incidence on said Fresnel screen.